BULB DISCUSSION BOARDS > Modern Electric Lighting

Strange phenomenon - evaporated tungsten

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Yoshi:
Anyone good in chemistry here? I am puzzled about what just happened! Here's the story:

I took several non-functional C7 bulbs from a chandelier (I was replacing them with new ones). These bulbs had a band-like silver coating on the inside that was caused by use. I kept a few of these bulbs because I was going to use one as a tool for a special test (but that's another story...). I broke only the top part of one bulb, and left it on my desk for about a week. I just looked at it right now, and... The silver coating is COMPLETELY gone!! The whole bulb became crystal clear. I did not clean it, and I'm sure nobody else did. Why did this happen?


-Yoshi

James:
Yoshi,

It is almost certainly an oxidation reaction.  In miniature lamps of this kind, it is standard practive to apply a very thin layer of sodium aluminium fluoride (cryolite) onto the inner surface of the glass.  This ensures that when evaporated tungsten condenses on the glass, it appears less black than usual, and won't absorb so much light.  You can still see it, but much less than if there had been no cryolite coating on the glass.

I don't know precisely what chemical reaction will have been going on, but basically the tungsten (or its reaction products with the cryolite coating) will have reacted again with the air to form something of lighter colour, which you cannot see so easily.  Tungsten itself is greyish, and its principal oxide is a pale cream colour which is harder to see.

Best regards,

James.

Yoshi:
Thank you James! Very interesting explanation, as always!      

Stan:
Hi friends. I want only tell to you, that cryolite is practically absolutely stable on air, that it is also mineral, a source of aluminium (the localities e.g. Groenland, etc.). It use also in aluminium technology, as metal cleaner and in liquid (melted) form it is used as cover of surface aluminium bath - a protection against oxidation and make easy melting process. therefore I think, that the layer was something other than cryolite. Stan

Stan:
James had right, certainly. I saw tungsten oxidation products on inner wall also in other colors - greenish but mainly yellowish.
Any Russian bulbs used also metal iodides in very thin deposition on the filament. Then is possible using of greater amount (as mistake) and through oxidation it don?t makes fully translucent oxide layer. Stan

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